1. Field of the Invention
This invention relates to a method of descaling metal sheets, especially titanium and stainless steel sheets. In particular, it relates to a method of descaling such metal sheets which includes the step of subjecting the metal sheets to anodic electrolysis in an electrolytic solution.
2. Description of the Prior Art
Metal sheets, such as titanium sheets and stainless steel sheets, are generally produced by the steps of hot rolling or hot stripping, cold rolling, annealing and skinpass rolling. Hot coils obtained from hot strip mills as well as sheets after annealing have scales consisting essentially of oxides (e.g. TiO.sub.2, Fe.sub.2 O.sub.3, FeO, Cr.sub.2 O.sub.3 etc.) and surface defects inevitably formed during rolling steps. When they are rolled as they are, the surface of the product sheets is impaired and the products lose their value. Therefore, descaling treatment is essential prior to cold rolling of hot coils or skinpass rolling following annealing. These scales are stable or insoluble in usual pickling solutions, such as hydrochloric acid and sulfuric acid, and therefore satisfactory descaling cannot be achieved with such usual pickling solutions.
The conventional method exclusively used for descaling titanium sheets includes the step of immersing titanium sheets in a mixed acid solution containing hydrofluoric acid and nitric acid in concentrations of 1-4.5% and 3-20%, respectively, whereby scales are dissolved and removed. Thus nitric-hydrofluoric mixed acid solution is highly capable of dissolving titanium oxides and achieves such a high efficiency that the descaling rate amounts to about 3 microns per minute. However, the nitric-hydrofluoric mixed acid solution is very strongly corrosive and thus requires a treatment plant with a high degree of anticorrosiveness treatment. For these and other reasons, the production line has to bear a heavy burden with respect to the cost of production. Also, the waste solution poses a problem of environmental pollution.
For descaling stainless steel sheets, it has been proposed to immerse the sheets in a variety of mixed or combined acid washing solutions, such as a sulfuric acid-sodium chloride solution, a hydrochloric-nitric acid solution, a sulfuric-hydrochloric acid solution, a sulfuric acid-ferric sulfate solution, hydrochloric acid-ferric chloride solution and a hydrofluoric-nitric acid solution. Especially, a mixed acid solution containing hydrofluoric acid and nitric acid in concentrations of 1-5% and 3-10%, respectively, has exclusively been in practical use in treating stainless steel sheets by immersing the same therein for the purpose of descaling because the solution achieves such a high treatment efficiency that it is highly capable of dissolving oxides and achieves a treatment efficiency as high as about 2.3 microns per minute. However, as mentioned above, the nitric-hydrofluoric acid solution is highly corrosive and thus requires treatment equipment having a high degree of anticorrosiveness and the waste solution poses a problem of environmental hazards. Moreover, an adequate bath control is required for maintaining the treatment velocity at a constant level, but such control is burdensome. Consequently, this method of descaling is not an adequate one to be employed in a continuous production line.
Furthermore, K. Hata et al. describes the present status of electrolytic descaling of stainless steel using neutral electrolyte [(The Hitachi Hyoron, vol. 58, No. 9 (1976-9)], and S. Elsner discloses a process and apparatus for continuously electrodepositing metal of relatively uniform thickness on a moving metal substrate at extremely high plating rates (U.S. Pat. No. 3,619,383).